Polyaspartic acid has been produced by thermal polymerization of L-aspartic acid which involves heating the acid to a temperature in the range of from about 200.degree. C. to about 400.degree. C. Water is driven off as the acid polymerizes to form polysuccinimide. The imide is easily converted to polyaspartic acid by basic hydrolysis. Early interest in such processes related to theories for formation of prebiotic polypeptides. For the purpose of testing such theories laboratory experiments used powdered L-aspartic acid, usually packed in the bottom of a flask which was then heated below the melting point of the acid. Such reactions were slow and took place over many hours. One such example is reported by Kokufuta et al. in Bulletin of the Chemical Society of Japan Vol. 51 (5) 1555-1556 (1978) "Temperature Effect on the Molecular Weight and the Optical Purity of Anhydropolyaspartic Acid Prepared by Thermal Polycondensation." The structure of anhydropolyaspartic acid has been thoroughly investigated such as by J. Kovacs et al. in J.O.C.S. Vol. 26 1084-1091 (1961).
In recent years many utilities have been suggested for anhydropolyamino acid. Such polyamides have been suggested as potential drug carriers by Neuse et al. in Die Angewandte Makronmolekulare Chemie 192 35-50 (1991) "Water-soluble polyamides as potential drug carriers." They have also been tested as scale inhibitors with respect to natural sea water and calcium sulfate in particular by Sarig et al. as reported by the National Council on Research and Development (NRCD 8-76, Seawater Desalination 150-157 (1977). Polyaspartic acid has been well known for its ability to disperse solid particles in detergent formulations, having been mentioned as a dispersant in numerous patents, a few of which are U.S. Pat. Nos. 4,363,797; 4,333,844; 4,407,722 and 4,428,749. Also, as described in U.S. Pat. No. 4,971,724 to Kalota et al., it has been discovered that compositions comprising polyamino acids such as aspartic acid, when ionized at alkaline pH, effectively inhibit corrosion of ferrous metals in the presence of aqueous medium. Various derivatives of polyamino acids have also been made wherein attributes have been supplied by groups attached to reactive sites on the molecule. One such example is disclosed in U.S. Pat. No. 3,846,380 to Fujimoro et al.
Because of the various impending potential utilities of anhydropolyamino acids, interest in processes for preparing such compounds in large volume, particularly polyaspartic acid, has increased. This interest has resulted in several recent patents being issued which are directed to fluid bed systemic; in particular, U.S. Pat. No. 5,219,986 to Cassata. Other such patents are U.S. Pat. Nos. 5,057,597 and 5,221,733 to Koskan and Koskan et al. respectively. More recently, patents have issued covering a process for preparing the polysuccinimide by means of tray driers such as a rotary tray drier in U.S. Pat. No. 5,319,145 to Palk et al. and an indirectly heated tray drier in U.S. Pat. No. 5,315,010 to Koskan et al. When phosphoric acid is employed in these processes the aspartic acid undergoes polymerization to form the polysuccinimide, forming a course powder containing lumps of up to about two and one-half centimeters in diameter. Prior to subsequent processing the course powder must be treated to remove the lumps. Furthermore, in the use of an acidic catalyst such as phosphoric the powder undergoes a tacky phase which makes the powder difficult to handle in these dryers.
It has been often noted in the literature that the color of the polysuccinimide is related in rough manner to the time/temperature relationship in its production. For example, the color of the product from processes employing relatively longer reaction time under elevated temperature produces more darkly colored polysuccinimide than is produced with shorter reaction time and lower temperatures. A typical teaching of this phenomenon is found in a publication entitled "Temperature Effect on the Molecular Weight and the Optical Purity of Anhydropolyaspartic acid Prepared by Thermal Polycondensation" by Kokufuta et al., Bulletin of the Chemical Society of Japan, Vol. 51, pp. 1555-1556, 1978.
Many efforts have been made to lower the time/temperature relationship by employing the above noted dryers. However, the polysuccinimide produced by such processes possesses an undesirable color. The color of the polysuccinimide is transferred to the water soluble salt upon hydrolysis of the initial product.
It has been reported in the literature that the use of acidic catalysts such as phosphoric acid reduces the color of the resulting polysuccinimide. It is believed that phosphoric acid increases the reaction rate and therefore reduces the amount of time required at high temperature for completion of the reaction. While color of the polysuccinimide produced in a solution of phosphoric acid is improved, the use of large amounts of acid is inconvenient. Another attempt to employ liquid media for the L-aspartic polymerization process is found in U.S. Pat. No. 5,371,179 wherein the use of poly(alkylene glycols) is employed.
In one attempt to overcome the color problem it has been found that the polyaspartate formed by thermal polymerization followed by alkaline hydrolysis is treated with bleach in water solution. This treatment is reported to decolorize the solution of polyaspartate as noted in U.S. Pat. No. 5,292,864 to Wood, et al. However, in many instances the color of the polysuccinimide becomes a disadvantage as this initial product is employed without first converting it to the water soluble polyaspartate salt as was done in the above noted patent to Wood et al. For example, in U.S. Pat. No. 5,266,237 to Freeman et al. and Australian Patent AU-A14775/92, the polysuccinimide is added directly to other ingredients to form a detergent composition. The color of the polysuccinimide, particularly in detergent applications, is desirably white. Thus the method of decolorizing the water solution of the salt is not useful in preparing desirable compositions of the above noted patent to Freeman et al. which incorporate the succinimide. Accordingly, there is needed a convenient process for the production of polysuccinimide which has acceptable color for detergent applications without the need for decolorization. More convenient liquid media are needed for large scale production of the polysuccinimide which has very little color.